Heat oven to 200C/180C fan/gas 6. Line the base and sides of a 20 x 30cm baking tin with 2 criss-crossed strips of baking parchment. Unroll the pastry and sit in the centre of the tin so that it lines the base, and evenly comes up the sides a little. Lay a sheet of greaseproof paper (not parchment) on top. Add some baking beans, then bake for 20 mins. Remove the beans and paper and bake for 10 more mins until pale but biscuity, then lower the oven to 180C/160C fan/gas 4.
Mix the golden syrup, breadcrumbs, oats, lemon zest and juice and the eggs. Tip into the pastry, spread evenly, then bake for 40 mins until golden brown and set. Cool, then slice into bars. Will keep in a tin for up to five days.